Method of reducing the concentration of silica in sodium aluminate solutions



Aug. 22, 1950 E. P. FLlN-r ETAL. 2,519,362

METHOD OF REDUCING THE CONCENTRATION OF' SILICA IN SODIUM ALUMINATE SOLUTIONS Filed Aug. 4. 1943 5 Sheets-Shea?I 1 ffOmEY LANS/N6 5Q'WELL5 Aug. 22, 1950 E. P. FLlN-r ETAL r METHOD 0F REDUCING THE CONCENTRATION 0F SILICA IN SODIUM ALUMINATE SOLUTIONS Filed Allg. 4, 1943 5 Sheets-Sheet 2 EAMS NW9"f5/ITER .4v-"Tammy ANSINGPWELLS Aug. 22, 1950 E. P. FLlNT ETAL METHOD 0F REDUCING THE CONCENTRATION OF SILICA v 5 Sheets-Sheet 3 IN SODIUM ALUMINATE SOLUTIONS Filed Aug. 4, 1943 Aug 22 195%E E P FLINT ETAL 2,519,362

THOD OF REDUCNG. THE CONCENTRATION OF SILICA IN SODIUM ALUMINATE SOLUTIONS Filed Aug. 4, 1943 5 Sheets-Sheet 4 KAoL /N INVENTORS 4 BY /NAR FL/NT gwz i I L50 SHAPT/S rToR/EY LANS/N6 5. WELLS Aug. 22, 1950 E. P. FLINT ETAL 2,519,362

METHOD oF REDUCING THE CONCENTRATION 0E sTLIcA IN SODIUM ALUMINATE SOLUTIONS v` Filed Aug. 4, 1945 5 Sheetls-Sheet 5 2 ATTaPh/EY Patented Aug. 22, 1950 METHOD `OF REDUCING THE `CON-CENTRA TION OF SILICA IN SODIUM ALUMINATE SOLUTIONS Einar P. Flint, Washington, D. C., Leo Shartsis,

Bethesda, Md.. and Lansing S. Wells, Washington, D. C.

Application August 4, 1943, Serial No. 497,346

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 4 Claims.

The invention herein described may be made and used by or for the Goverment of the United States Without payment to either of us of any royalty therefor in accordance with the provisions of the act of April 30, 1928 (ch. 460, 45 Stat. L. 467).

This invention relates to alkaline processes for extr-acting alumina, as from W-grade bauxites and clays, yand aims generally to improve the same.

A principal object of the invention is to provide processes of the 4class described adapted to yield products of suicient purity to meet the requirements for alumina to be used in the manufacture of aluminum by electrolytic reduction.

A further object is to provide a. process which may be wholly independent of the use of highgrade lbauxites.

An additional important object of the invention is to provide an improved method of desilicizing a solution Containing silica, alumina, and soda, by adding to the solution a sodalitictype or sodalitic salt-forming reagent to cause the silica to precipitate as a sodalitic-type salt of low solubility compared to the form of precipitate obtained in the ordinary Bayer or Pederson or like processes, preferably with seeding.

Other objects and advantages of the invention will be apparent from the following general and detailed descriptions of particular embodiments illustrating preferred applications of the invention to extraction of alumina from low-grade bauxite, recovery of alumina and soda. from the residues remaining after extraction of the bauxite in accordance with the present invention, and extraction of alumina from clay in accordance with the present invention.

Oiur invention, as above mentioned, may be applied in various alkali-extraction processes with desirable results. For example, in the ordinary Bayer process of extracting alumina from bauxite, the bauxite, which contains the major part of its alumina in the form of gibbsite (AlzO3.3I-I2O) or dia-spore (A1203.H2O) and the major part of its silica as alumino-silicates, particularly kaolinite (AlzOaZSiOaZHzO), when digested with sodium hydroxide solution at a temperature of about 150 C. yields a solution of sodium aluminate (Na2O.Al2O3) and a red mud. The'sodium aluminate (NazO.Al2Oa)A solution is 2 cooled, seeded with gi'bbsite (A12O3-3H2O), and crystallized t0 the equilibrium of gibbsite solubility at the temperature of final crystallization. The gibbsite is then Calcined to A1203.

'The silica content of the bauxite, on digestion, is believed to be converted for the most part, to hydrated sodium aluminum silicate (probably of the formula Na2O.AlzOs.2Sio2.2I-I2O), which has a relatively low solubility in the solution and is renewed in the red mud. The silica which remains dissolved `corresponds to the solubility of the hydrated sodium aluminum silicate in the digestate and is largely retained in solution during the crystallizing out of gibbsite, which increases the causticity of the solution. When the NaOH in the digestate solution is neutralized with CO2, very .careful pH control must be exercised to prevent precipitation of silica along With gibbsite, if the precipitation is carried anywhere near completion.

In the application of the applicants present discovery in this type of process, a considerable quantity, preferably a large excess at least equal to the concentration of the sodium oxides present, of a sodalitic-salt forming reagent herein generally termed N azR, as sodium (or potassium) f nitrate, -chloride, -bromide, -sulfate, carbonate, and like salts, is added to the material being digested (a non-carbonate salt, herein termed NarR being preferred, as will hereinafter appear), with the result that the silica compound in the red mrud is rendered sodalitic (e. g. of the probable formula Na2O.Al2O3.2SiO2.%NazR) and is much less soluble than the non-sodalitic compounds such as NazOaAlzOaZSiOaZHzO, so the silica concentration in the solution is reduced. This step is advantageous, and is particularly ydesirable when digesting by boiling rather than autoclaving, as the formation of a sodalitic red mud for our invention during the boiling operation leaves so little silica in the solution that it becomes feasible to carry the CO2 neutralization of NaOH, for throwing do-Wn the alumina as gibbsite, much further in all cases without obtaining undue amounts of silica in the product. When the digestion was made in the presence of suitable concentrations of the monovalent sodalite-forming salts, more especially NaCl and NaNOa, the precipitation of gilbbsite may be carried practically to completion, as could not have been predicted prior to our research,

In the recovery of alumina and soda from the residues of the ordinary Bayer process, the red mud is calcined with lime to convert the sodium aluminum silicate to relatively inert dicalcium silicate and sodium aluminate, which can be leached out with water at a temperature Well below boiling.

We contemplate that our discovery may be ernployed in this connection by extracting the sinter produced from the red mud of the ordinary Bayer process with a solution containing a concentration of sodalitic-salt forming reagentsubstantally as great as or greater than that of total NaOH, and then boiling the extract with a seed charge of socialite-type compound to bring'down its silica content sodalitically.

We prefer however, when applying our discovery to treatment of red mud; to apply it, tov sodalitic red mud obtained by our principal extraction, by calcining the sodalitic red mud with lime, and then leaching it with the salt solution from our principal extraction (after precipita'- tion of alumina from ity-this salt solution being one in which the sodaliticesalt forming reagent is. preferably a salt NaeR.' other than the carbonate, and being used at about 5 6" C. or below-y followed by boiling with a seed charge of the sodalite-type compound to bring down the silica as a highly insoluble sodalitic compound, after which the hydrated alumina may be precipitated With- CO2, leaving a solution of the salt, e. g. sodium chloride and sodium carbonate which can be causticized with Ca(OH) 2 and returned to the first or principal extraction step of our process.. Thus, in this second application of the invention the sodalitic-salt forming reagent (NaeR') 'again functions to convert the silica in solution to a sodalitic type compound, thus again enabling the 4COQ precipitation to be carried practically to completion without obtaining excess silica in the product.

i Our invention is also applicable inthe extraction ci alumina from otherV aluminiferous material, as in the lime-sinter methody of extracting alumina from low-grade bauxite or clay (kaolin or the like). In the ordinary lime-sinter process, for example, the crude low-grade bauxite or elay i-s sintered with lime to formcalcium silicate and calcium aluminat'e, which sinter -is extracted with NazCOz solution yielding a 'solution of sodium aluminate with dicalcium silicate and calcium carbonate as residues. Following a dev`silication treatment of the sodium aluminate solution its alumina content is precipitated as gibbsite by means of CO2.

In applying our discovery to -such a process the sodalitic-salt forming reagent employed (NaR) is one other than the carbonate, because in the presence of dicalcium silicate, NazCOs in such large excess as is required to eiiectually desili- Vcize by formation of the alkali carbonate ,soda- Y litio-compound, will decrease the yield of lsodium aluminate solution, apparently as a result of its partially decomposing the dicalcium silicate. Accordingly, in any application of our process `in which dicalcium silicate is a factor, a non-carbonate reagent (NaIR) such as NaCl, NaBr, NaNOa or the like is doubly preferred. In the application of our method to a lime-sinterprocess, the reagent (NaeR') is usually included in the NazCOa extractant solution, its presence during the extraction being tolerated in order to be able to use the (N aeR) solution from the tail end of the process, as will appear hereinafter. The (NaeR) is, so far as advantage is concerned, inert during the extraction step (it is slightly disadvantageous as it slightly decreases A1203 extraction). It is put in at this stage so it will be present in the extract to convert the dissolved silica into sodalitic-type compounds removable by boiling with sodalitic type seed to reduce the silica befoi'e theV CO2 treatment of the residual liquor. In this connection we have determined that sodalite and similar sodalitic-type compounds are less soluble at boiling temperature, and that theCOz treatment is best carried out at near boiling temperature, and thus contemplate such conditions in the preferred form of this application of our invention.

It will accordingly be seen from the foregoing general description, that our invention provides a new manner of reducing the silica content of alkaline aluminiferous extracts per se, which in its broader aspects is not limited in application to any one particular extraction process, although in particular applications its inclusion results in new and useful improvements of the combinations or sub-combinations thereof.

.A more detailed understanding of the essential steps of the invention, its generic and specific aspects, and new combinations to which it contributes, will appear from. the following general and detailed description of illustrative embodiments, in connection with the accompanying drawings in which Figs. 8, 14 and l5 are flow sheets of illustrative embodiments of complete extraction processes embodying our invention,v with solids, liquids and gases shown respectively in chain-form, continuous, and dotted lines; while- Figs. -l to 7 and 9 to 13 are charts in the form of interpolated and extrapolated curves based on actual tests inthe critical regions, Showing preferred conditions for the several applications of our invention and the improved results achieved by such applications.

Referring first to Figs. 1-8 of the drawings, the novel features of the present invention are therein `described in` connection with a sodium hydroxide-bauxite extraction process, and as indicated in Fig. 8, reside principally in the composition of the solution used for extracting the alumina content of the raw material and in the method of recovering alumina and soda from the residues remaining after extraction. The extracting solution in the principal extraction step according to our invention consists of a mixture of sodium hydroxide and a silica depressing sodium salt. Various sodium salts, such as the chloride, nitrate, bromide, sulfate, or carbonate may be used, which in this description are represented as NaeR. When these salts are present in sufficient concentration the amount of silica dissolved from bauxites is much less than when the extraction is carried out with sodium hydroxide solution alone, -as in the Bayer process. The diminished concentration of silica in the presence of sodium salts -as abovel mentioned, is believed to be caused by the combination of these salts with sodium aluminum silicate to form very slightly soluble compounds -related to sodalite Ybe u2, and R may `be Cl, N03, S04, CO3, etc., and

is the Valence of the acid radical. Of the-salts mentioned above, sodium chloride and sodium nitrate are indicated by our research to be the A tioned, is preferably not a carbonate. The salt preferred would ordinarily be sodium chloride which is cheap and plentiful and unpredictably effects materially better desilication than most other salts except sodium nitrate. The extract is then desilicized by boiling it with a seed charge of synthetic sodalite, preferably of line grain and approaching the theoretical molar composition of the mineral. By this method of desilication, the present invention enables reduction of the amount of silica in solution to a few hundredths of a percent of the alumina concentration.

Modifications of the present new method may be used successfully for extracting alumina from clay or kaolin, as indicated in Figs. 9 to 14 herein, as well as for extracting alumina from low-grade bauxites and other aluminous materials (see Fig. The first step in the modified procedure involves burning the aluminous material with sufIicient limestone to convert its alumina to a calcium aluminate and its silica to dicalcium silicate. For mixing and burning the raw materials and grinding the product the equipment used in cement manufacture is suitable. The product is then extracted with a solution of sodium carbonate containing a relatively high concentration of sodium chloride, sodium nitrate, or some other sodium salt which in this instance, is preferably not a carbonate, for the reasons above mentioned, and is herein termed NazR. To prevent the formation of compounds of the sodalite type during'extraction, with consequent losses of alumina, soda, and sodium salt, the extraction temperature is preferably kept not much in excess cf 59 C. The resulting extract is then boiled with a seed charge of synthetic sodalite or double salt of the sodalite type. As mentioned in connection with the embodiment of Fig. 8, this treatment reduces the amount of silica in solution to a few hundredths of a percent of the alumina concentration. Alumina may then be precipitated from solution with carbon dioxide.

By the processes described herein high recoveries of alumina, having a purity comparable to that of the product obtained in the Bayer process, can be obtained from both low-grade bauxites and clays, without the use of any high-grade bauxite.

AReferring more specifically to the embodiment exemplified in Figs. 1-8, a detailed understanding of the method may best be gained from a consideration of the materials actually used and the procedures actually followed in our research.

The materials chosen for extraction were three Georgia bauxites of the compositions given in Table 1. Other materials used in the extractions droxide, sodium carbonate, sodium sulfate, sodiumbromide', sodium chloride, and sodium nitrate.-VIV

15 same salt used in the first stage in the case of a two-stage process and which, as above men- .Table Lfompositions of bauites l :Baume Oxide A 1 B 2 C 1 Percent Percent Percent 19. 19. 89 11.08 44. 14 50. 57 56. 18 9, 6l l. 26 1.18 2. 05 2. 52 2. 39 0. 02 0. 03 MgO 0.01 0.01 Ignition Loss 23. 58 25. 30 28. 38

` Total 99.18 99.57 99.25

l A-Georgia bauxite from Ledden property. 2 B-Georgia bauxite from Lindsay property. 3 C- Georgia bauxite from Hightower property.

The procedure involved boiling or autoclaving the finely-ground bauxite with a sodium hydroxide solution containing Various proportions of sodium salts. Extractions at boiling temperatures were made in iron containers tted with reiiux condensers and those at C. in a bomb-type autoclave having aV capacity of l liter. After a heating period oi 3 hours the residues were separated from the solutions by ltration, washed with water and the combined extracts and washings analyzed gravimetrically for SiO2 and A1203. In some cases the approximate concentrations of NaOH and Na2CO3 were determined by titration of an aliquot of the solution with standard acid. Operations were also carried out in which the alumina in solution was precipitated with carbon dioxide gas under various conditions. rlhe variations in composition of both the precipitate and the solution were followed by chemical analyses during this process. Microscopical examinations Were made of the precipitated alumina. The final products, after filtration, washing, and drying were analyzed for silica, alumina, and ignition loss, and in a few cases, for ferrie oxide and soda. Analyses for silica were always made on samples which were large enough to contain 5 to 10 g. of A1203.

Extractions were made both on a laboratory and small pilot plant scale. The solid residues from the pilot plant extractions were dried, analyzed, and further treated to recover the alumina and soda which they contained. This treatment consisted in mixing the residues withV calcium carbonate, heating to suitable temperatures, and extracting the ground product, with water, sodium hydroxide solution, or sodium chloride solution.

Studies were made of the composition and structures, as revealed by X-rays, of the solid' ous sodium salts were also investigated and compared with theresidues from the extractions of the bauxites.

To determine the molar ratios of Na2O (as NaOH) to A1203 in the bauxites that would give satisfactory recoveries of A1203, a number of extractions of the three bauxites were made at boiling temperatures. The volume of the extracting solution was 200 ml. in all experiments. The

- quantities of bauxites used and the compositions and after extraction are Table 3.-Laboratory extractions of baurztes withsoalum hydroxide and s'alt solutions-continued Bauxite used, g. per l. in Solug./1. sa Added gll Temp. in tion of- Recovery Ratio Extraction No. dgretes of A1205, SiOn to cn Percent A120 Kind Amt. Kind Amt. A1203 SiO: Percnt A 37. 0. 017 43. 5 0.05 A 50.1 0.016 63.1 0.03 A 49.1 0.048 62.0 0. A 48. 7 0. 010 V61. 5 0. 02 .4. 48. 4 0. 013 61. 1 0. 03

A 180 55 46.2 0.036 58.2 0.08 .4 180 66 49. 4 0. 009 62. 2 0. 02 A 180 dn 42. 6 0. 008 53. 7 0. 02 B 157. 5 54. 0 0.324 67. 9 0. 60 B 157.5 Nalo 53.7 0.177 67.5 0.33

B 157.5 do 52.5 0.147 66.0 0.28 B 157.5 do 55.5 0.130 69.8 0.24 B 157.5 dn 53.7 0.129 67.5 0.24 B 157. 5 51. 6 0.146 64. 9 0.28 B 157.5 Nance,--- 53.1 0.074 66.8 0.14

B 157.5 Naisolm 125 B6i11ng 53.6 0.108 67.5 0.20 B 157.5 dn 250 n 52.7 0.102 66.2 0.19 B 157. 5 dn 375 dn 49. 4 0. 080 62. 0 0. 16 B 157.5 -dn 500 dn 53.8 0.071 67.8 0.13 B 157.5 dn 250 150 53.0 0.053 66. 7 0.10 B 157.5 NaBr- 375 Bo11ing 47.7 0.076 60.0 0.16 B 157.5 NaC1 125 dn 51.3 0.169 64.5 0.33 B 157.5 d0 250 d6 51.2 0.113 64.4 0.22 B 157. 5 dn 375 dn 50. 4 0. 071 63. 4 0.14 B 157,5 65 500 an 50.5 0.056 63.5 0.11

B 157.5 dn 250 150 48.2 0.021 60.6 0.04 B 157.5 NaNo,.-. 375 B611ing 47.1 0.057 59.3 0.12 B 157.5 dn 250 150 51.8 0.018 65.2 0.03 C 142 Non@ Boiling,.-- 61.8 0.340 77.4 0.55 o 142 Nazcoam. 62.5 do 61.3 0.245 76.8 0.41

c 142 dn 125 dn 61.4 0. 233 77.0 0.38 c 142 do 250 -d5 61.8 0.212 77.4 0.34 o 142 -60 250 150 60.6 0.088 75.9 0.14 o 142 N501. 125 Boiling 59.2 0.102 74.4 0.17 C 142 dn 250 dn 56. 8 0. 069 71. 5 0.12 c 142 d6 375 do 52.6 0.032 66.2 0.06 c 142 65 250 150 56.0 0.024 70.2 0.04

Table 3 shows that the order of increasing ef- A number of extractions of the three bauxites iectiveness of the salts, in depressing the concenwere made in a small pilot plant. The quantities tration of silica in the extracts, is as follows: of materials used and data obtained in somerep- NazCOa, Na2SO4, NaBr, NaCl, NaNOs. Generally resentativ'e extractions are given in Table 4.

Table 4.-Pz'lot plant extractions of baurites tracts are considerably lower than those obtained dissolved salt was heated to boiling in a gallon at boiling, but for bauxite A, in the presence of digester provided with a heat-insulating jacket,- NaNO3, boiling appears to be ahnost as effective and the bauxite then added, after which the mix-- as treatment at 150 C. The purity of the alumina. tures were heated for 3 hours by passing steamin so-lution with respect to silica in the presence into the digester. Condensation of steam during of the Various salts is more clearly shown in Figthe extraction increased the volume of the soluures 2 to 7. tion somewhat. Continuous agitation was main- A slight decrease in the recovery of alumina, tained by means of a mechanical stirrer. Exas the concentration of added salt is increased, tractions'l, 2, and 3 of Table 4 were made at atis noticeable in Table 3. mospheric pressure and, extractions 4, 5, and 6 In a number of extractions, particularly those at approximately 30 lbs. gauge pressure (135 C.). carried out in the autoclave where good agita- At the end of the heating period the mixture was tion could not be maintained, low recoveries were filtered. The filter cake was then washed with sometimes caused by caking of the bauxite durhot water and the volume of the extract plus the ing extraction. Wash water was measured. Analyses of the resulting solutions were` made Vfor. Na2CQ3, NaOH, v A1203, and SiOz. The iigures given in Table 4, for recoveries of NazCOa, NaOH, and A1203, are based on the amounts of these materials found in solution. The proportions of SiO2 to A-I2Oatin the solutions, in percent, are plotted foitbauxite B in Figure 3, where the values are designated as Large Scale. It Willbenoted that4 the values are comparable to those obtained in the laboratory-scale extractions. .The Vpercentage recoveries of alumina were slightly -lower inthe pilotplant than in the laboratory extractions -because of less favorable operating conditions.

Complete, or almostcomplete, precipitationpf .As .this .differential precipitation continues the ratio of silica to alumina,insolutionincreasesto a greater extent than thatin the precipitate. However, if thefprecipitation of alumina is carried to.completion,practically'all of' the silica remainingin solution is brought down also. Hence itis -advantageousvto avoidthe precipitation of V.theiasttfraction of the alumina.

.Chemicalanalyses of the residues vfrom pilot .plantextractionsrof the three bauxites .Withso- -dium fcarbonate-sodium hydroxide f solutions are .givenin-Table 6. .Thefmolarratios oftheresidues, Aexceptfor-thosefrom bauxite lC, are yclose the alumina in the lpilot-plant extracts `was tothe proportions .l.ONa2O:1.0Al2O3:2SiO2.

,Tabled- Compositions of residues .from pilotplantetractions of bauites B t I Oxide Qornposition (ignition-free basis), percent MolarARatos auxl e 'Extraction No. designa? A `14. 77 29. 16 31. 69 18. 51 15x29 0. 8 1. 0Y 1.8 A 16. 41 29. 97 33. 94 19. 26 0. 39 0. 9 1. 0 1. 9 A 16. 32 29. 96 34, 00 19. 04 f 0:25 0. 9 1. 0 1. 9 B 23. 62 32, 36 36104 7: 98 1. 15 1. 0 1.9 B y21.55 34. 4()` 35. 74 6. 52 1 1; 98 1. 0 1. 0 1; 8 C 16.45 38. 38 `28.` 04 7. 81 l. 9. 19 0. 7 1. 0 1. 2 C 15. 62 38. 11 31; 58 9. 77 1 4282 '.0. 7 1. 0 1. 4

ljCaO derived principally from-CaCO; added asfilter aid.

eiected by means of .carbon dioxide that was passed slowly into the solutions which were heated to boiling and kept in continuousagitation with steam.

The precipitated material was filtered, washed with hot water and dried overnight at 130 C.

Microscopical examination showed that .the preparations consisted of well-developed prismatic .crystals ,having .the `refractive indices of gibbsite.

.Chemical analyses ,of Ythe gibbsite .preparations obtained yfrom the extracts .listedin Table 4 showed that their ratios of silica to alumina were almost identical with those `of the `original solutions.

The possibility .of securing a diierential precipitation of alumina and silica by controlling the pH of the solutionwas investigated. For-this purpose an additional pilot plant extraction was made. steam, CO2 .was passed in ata slow'rate. Samples of the precipitated alumina and of ithe solution were withdrawn at intervals and .analyzed for A1203 and S102, as well as for measurement of the pH of the solution with a hydrogen elec- After heatingthe solution to boiling with v.In four instances (extractions2 and 3 with `b:a.uxite:A andextractions 6 and .'7 with bauxite C) the `residues were firstdried at C. and the .CO2 Acontent of Veach sample and its loss-on- 'ignition at 950 iC. .were thendetermined. 'The .content of H2Owastaken.as .the difference between the CO2 combined with the CaO (CaCO: added as a lter aid), the Na2O required to combine with .the remaining ,COzas NazGOa Was-.calculated and -this .value .of .NazO -was subtracted from .the .total ,of Nago. .The following molar ratioswere obtained.

Molar ratios Extraction No. of

Table 6 NazO A1201 SiO: 4 NazCOs H2O Numerous 4calculations 4based on the alumina and silica contents of the residues from laboratory-scale extractions and the losses in soda trode. The results are given in Table 5. occurring .during these extractions likewise in- Tablc 5.-Precipitationof alumina from Aextract of bauxite A o mposition or Solution H fs I Rfof Sigi to 'v p 0 0uz ainre- Y n Sio, tion cipitated 211101, Remarks* A: g.l1. S102 g./1. i Per Cent 12.8 0.0428 .0.33 13.04 (l) 11.1 o. 039s o. se 12.20 (2) 9.1 o. 033s o. 37 11. sa o. 1s (1) 7. a o. 0285 o. a9 12. 4o o. 21 e) 1. 9 o. 0.125 o. es 11. e4 o. 24

l Solution at start. `1 After passing in CO2 to effect slight precipitation.

3 After passing m CO2 to eect partial precipitation.

'4 CO2 addition stopped but alumina precipitation continued overnight.

5 Afterpassing in CO2 to precipitatemost of alumina.

The data in Table`5 .show .that the purity .vof ther precipitated aluminaA with respect .tosilica dicated approximately the .same :NazO A1203 :SiO: molar ratios. The residues must .also contain carbonate which disappears from solution during the extractions.

Accordingly, steps were taken to synthesize a sodium aluminum silicate carbonate compound of a composition similar to that of the residues. The following mixtures of gibbsite and silica gel were boiled with solutions of Na2CO3 and NaOH similar to those used in extracting the alumina from the bauxites.

Mix- Mixture ture #l #2 H2O, ml 400 400 NazCOz, Q 100 100 NaOH, g 50 50 A1203.3H20, g 13 26 Silica gel, e 1l. 4 11. 4

The mixtures were ltered and the residues washed and dried to constant weight at room The NazOzAlzOa S102 :NazCOs H2O molecular ratios are 0.98:1.00:2.00:0.14:2.42 and for samples 1 and 2, respectively.

An X-ray diraction pattern of sample 1 showed that it had a structure similar to that of the mineral sodalite,

In order to determine whether or not a sodium aluminium silicate compound of similar composition but containing no sodium carbo nate, would also have an X-ray diiTraction pattern similar to that of sodalite, the preparation of such a compound was attempted. In this experiment a mixture of g. of SiOz, in the form of silica gel, and 3-.1 g. of gibbsite was heated in an autoclave with 600 ml, of solution containing 90 g. of NaOH at 150 C. for 5 days. The resulting product was ltered from the solution, washed with hot water and dried in air. It had a composition represented by the molar ratios 1 17NazO: 1.00Al203 1.968102 2.11-120. This material had an X-ray diiraction pattern which was distinctly different from that of the preparation containing sodium carbonate.

These investigations indicate that the decreased. concentrations of silica obtained in sodium aluminate solutions in the presence of sodium carbonate are probably caused by the formation of a compound similar to sodalite and having a lower solubility than that of the hydrated sodium aluminum silicate formed in the absence of sodium carbonate. They raised the question also as to whether other sodium salts might form compounds of a type similar to sodalite, but afforded no basis for predicting whether such salt formed compounds would have desirably loW solu-bilities, there being no known data on the solubility of such compounds under the conditions here dealt with. Hence sodium chloride, sodium bromide, sodium nitrate, and sodium sulfate were substituted for sodium carbonate in the extractions summarized in Table 3 and Figures 2 to '7. As noted above, the results showed that lower concentrations of silica were obtained in the presence of these salts.

Preparations of the compounds containing sodium chloride, sodium bromide, sodium nitrate, and sodium sulfate were made in the autoclave at C. The X-ray diffraction pattern of the double salt of sodium aluminum silicate with sodium chloride was identical with the pattern given by the mineral sodalite. The pattern vof the bromide compound was almost identical with, and that of the nitrate compound very similar to, that of sodalite. As already stated the double salt with sodium carbonate also has a pattern similar to that of sodalite. However, the double salt with sodium sulfate has a pattern which diilers from that of sodalite and also from that of the preparation of sodium aluminum silicate containing no added salt.

There are four minerals which mineralogists have recognized as belonging to the sodalite group. These, according to N. H. and A. N. Winchell, Elements of Optical Mineralogy, pt. 2, 3rd ed., John Wiley & Sons, N. Y. (1931), are the following:

Naturally-occurring minerals, corresponding to the carbonate, nitrate, and bromide double salts, are as yet unknown It appears probable, however, that a considerable nulnber of double salts of the sodalite type are capable of existence, in which Cl may be replaced by a variety of acid radicals.

The preparations, on which the X-ray diffraction patterns had been obtained, were analyzed. The molar ratios, computed from the analyses, were as follows:

It will be noted that all of these preparations contain water of hydration and less sodium salt than corresponds to the reported compositions of the sodalite minerals. However, as mentioned previously, the X-ray diffraction patterns of the complex double salts, with the exception of the sulfate compound, were very similar to that of naturally occurring sodalite. This suggests the likelihood that a solid solution series between a hydrate and the respective sodalite type end member, is the actual mechanism producing the effect of, and herein termed, a sodalite-type compound.

A number of X-ray diffraction patterns were also made of the residues remaining after extraction of the bauxites with NaOH alone and with NaOH-salt solutions. In all cases these patterr-1s indicated that the residues had the same structures as the corresponding synthetic double salts.

An analysis was made of the residue from a laboratory-scale extraction of bauxite B with a sodium hydroxide-,sodium chloride solution. This residue had the following composition:

The corresponding molar ratios are 0.90Naz: LOOAlzOs :1.74Si0210-46NaCl These ratios do not diier much from those found for the synthetic preparation which were as follows: 0.96Na20:1.00Al203:l,9'7SiO2:D.53NaC1.

Summarizing the results of the X-ray and compositional investigation of the bauxite residues and of the synthetic double salts, it appears that the structures of the products containing sodium carbonate, sodium chloride, sodium bromide, and sodium nitrate are very similar to that of the mineral sodalite and that all may therefore be termed sodalitic or of sodalite type. The relatOIlShp to sodalite of the preparations containing sodium sulfate has not yet been definitely ascertained. These preparations may have the related structure of noselite. Additional X-ray studies of these compounds will be necessary to establish their structures more closely. Their formation is intended to be included, however, in the .broader aspects of our invention.

A, large number of experiments were performed to investigate the recovery of NazO and A1203 from the residues left after extraction of the A1203 from the bauxites. As discussed in the previous section, these residues contained soda. alumina, and silica in molar ratios approximating 1:112. The residues were mixed with calculated quantities of calcium carbonate and burned at various temperatures between 1000 and 1400" C. The object of this treatment was to break down the insoluble sodium aluminum silicate complex by combining the silica as relatively inert dicalcium silicate, leaving the soda and alumina in the form of soluble sodium aluminate. After grinding the products they were extracted with water, solutions of sodium hydroxide, sodium carbonate, or sodium chloride, or with combinations thereof.

Most of the extractions were made on burns prepared from the residues of bauxites B and C. These bauxites contained only small amounts of FezO: plus TiOz, while bauxite A contained a relatively large amount. Because of uncertainty as to the state of combination of F'e203 and TiOz in the presence of NazO and CaO, the addition of lime was calculated only on the basis -of the silica present. It was determined that the CaO/SiOz molar ratio is rather critical and if the optimum value of 2.0, for bauxies B and C, was departed from the extraction of both A1203 and NazO was sharply reduced. The optimum Ca0/Si02 ratio for bauxite A was not determined.

The extractions were performed at temperatures between room temperature and boiling for times ranging from one minute to two hours. When the sinters having a CaO/SiOz molar ratio of 2.0 were extracted with water, up to 95 percent of the NazO and percent of the A1203 were extracted. The presence of large amounts of NaCl in the extracting solution reduced the recovery of A1203 by a few percent. Further reductions occurred as the sodium carbonate Iconcentration of the solution increased, especially as it approached saturation.

Considerable SiOz was found in the extracts, sometimes amounting to 3 or 4 percent of the A1203 in solution. However, by boiling the solutions that contained suitable concentrations of NaCl (approximately 200 g. per liter) with seed charges of synthetic sodalite, the S102 concentration could be reduced to a few hundredths of a percent of the A1203 concentration. This decrease in SiOz was accompanied by corresponding reductions in the NazO, A1203, and NaCl contents of the solutions as required by the resulting formation of additional sodalite.

An illustrative process for extracting alumina from low-grade bauxites, employing the discoveries and method described in the preceding sections, is set forth in the accompanying flow sheet (Figure 8). In this flow sheet no attempt has been made to portray all of the various possible modifications of the separate steps. Furthermore, the flow sheet has been simplified by excluding from consideration constituents of the bauxite such as FezOs and TiOz, which are effectively separated from the recovered alumina and are discarded in the residues. The A1203 and SiOz contents of the bauxite, given in the flow sheet, are approximately those of bauxite B.

The values given in the flow sheet for the quantities and compositions of the materials at various stages of the process are based both on laboratory-scale and small pilot-plant results.

As shown in the flow sheet the finely-ground bauxite is digested for a suitable time with a sodium hydroxide-sodium salt solution, either at the boiling temperature of the solution at atmospheric pressure or at a higher temperature and pressure. The use of this extracting mixture constitutes one of the novel features of the process. In the presence of suflicient concentrations of sodium salts the amounts of silica dissolved from bauxites by the extracting solutions are greatly depressed because of the formation of insoluble silicate compounds of the sodalite type. Various sodium salts may be employed in making up the extracting solution, such as the chloride, nitrate, sulfate, bromide, or carbonate. Sodium chloride is one of the most effective of the salts and, since it is cheap, its use is indicated in the flow sheet.

After completion of the heating period in the digester, the mixture is filtered and the residue washed with water very low in silica; indicated in the iiow sheet as silica-free water. No attempt has been made in the flow sheet to specify the quantities of wash water to be used in various stages of the process since these quantities will vary with the scale of operation and may readily be determined by those skilled in the art. The residue is reserved for further treatment to recover most of the alumina and soda which it contains.

In our process the alumina may be removed from the combined extract and wash either by seeding with gibbsite, if the Nago/A1203 ratio is not too high, or by precipitation with carbon dioxide. The latter method is illustrated in the flow sheet where the kiln gas, after scrubbing, is shown as the source of carbon dioxide. By passing the kiln gas into the solution in the presence of a seed charge of gibbsite the major part of the alumina is precipitated when the sodium hydroxide in excess of sodium aluminate has been neutralized. The remainder of the alumina may be precipitated by continuing the slow addition of carbon dioxide. However, if the proportion of silica to alumina in the originalvextract was greater than 0.1 percent it may not be desirable to precipitate the alumina completely.

As shown in the flow sheet the residue from the initial extraction contains soda, alumina, silica, and sodium chloride in proportions approaching those of the mineral sodalite. It also `contains the relatively inert constituents of the bauxite such as FezOs and T102. Finely groundcalcium carbonate (limestone) is added to the residue to give a CaO/Ei02 molar ratio of 2.9 and the mixture then heated toa suitable temperature to bring about clinkering or sintering. After grinding, it is extracted with the solution which remained after precipitation of the alumina in the rst stage of the process. A small amount of lime is added to this solution tocon've'it some of its sodium carbonate to sodium hydroxide. The temperature of extraction should' probably not exceed 50 C. and the time required will vary with the character of the sinter. l

On completion of the extraction the mixture is ltered and the residue washedmand set aside. This residue with certain modifications can be used as a raw materialforY themanuiacture of Portland cement. The solution; consisting of combined extract and washings, is then desilicized by boiling it with a seed charge of synthetic sodalite. This method of desilica'tion by means of sodalite seed, constitutes anov'el feature ofu the present process. The seed charge, together with the additional sodalite iormed'byy desilicas tion of the solution, is then ltered oil and washed. A quantity of the sodalite corresponding to the original seed charge is used in subsequent desilications and the excess is treated, along with the next batch of residue, for' recovery of alumina and soda which it contained.

Because of obvious advantages in the design' of plant equipment for operation atA atmospheric pressure over that required at elevated pressures, the desilications in our research were made, for the most part, in open containers. It should be pointed out, however, that desilicati'n at elef-Y vated temperatures' arid pressures the more effective a's' indicated by comparison of Figures 5 to 7 with Figures 2 to 4.

As in the irs't stage4 of the process, the alumina is precipitatedv from solutionY as' gibbsite. The

gibbsite is filtered' from' the solution, washed, and" combined with the gibb'site 'obtained in the original extraction. Sodium carbonate is added to the` remaining solution which is then concentrated by evaporation, causticized, and used for the extraction of more bauxite in the iirst stage of the process.

An outstanding advantage ofthi's process, over other processes used heretofore, is that it is entirely independent of the use of high-grade bauxite at any stage of the extraction.

Referring more specifically to the embodiment exempliiied in Figs. 9-14, for detailed understanding we will again consider the materials actually used and the procedures actually followed in our research.

The starting materials in these instances were a Georgia kaolin and a ground IndianaA limestone of the following compositions:

Technical or C. P. grades of sodium ca rbonate,y sodium chloride, and sodium hydroxide were used in making vup the extracting'Y solutions'.

A quantity of the ground kaolin, suicient to contain l!) lb. of A1203, was mixed withthe grour'i'df limestone in such amount that after allowing for the CaO and A1203 required to form and ZCaOTiOz with the Fe2Q3 and TiOz;y present, the composition oi the residue" ii/"ildV lije on the join' between 2CaO.SiO2 and 5CaO.3Al2O3 in the ternary system CaO-Al2O3-Si02. v ,v v

This material wasv burned at 173001350 C'. for 5y hours and allowed to cool slowly. Complete dusting ofA the product occurred. Its composition was as follows:

l Per cent A1203 t; F20 0I95 0.32V SiO2 22.921 Q30 56.80 Mg() 0;'72'

This material consisted largely of particles of 30-50 microns in diameter.4 Some experiments were made on the unground si'nter and others on the rris'iterial after it had been ground to an aver-i age'particle size of 5 microns.

extractions of the' sinter were" made'witn s1u= 1 tions containing sodium carbonate orniixtures of sodium carbonate and sodium chloride. The extracts that contained sodiumfchlorid'e weredei: silicized in accordance' with our invention by ,b'oi

ing them with seed char'gs'or synthetic sfidante:

iime-kaolin sinter, a number of extractions were' made on a small-batch sinter of the sam'e composition. This was prepared by burning g. quantities of the original mix in aA mufile at lflfOU" C. until the product gaveno test for' freelime.

In order to ascertain yields and the purity of' the extracted alumina with respect'to silica, arialyses of the extracts were made for A1203 and SiO2 and, in some cases, for Naz() and NaCl. The' re` lsults are expressed inr terms of the volume of combined extract and Wash.

The extractions listed in Table 7 were madeon- Sodalite preparations" the small-batch sinter. The volume of the extracting solution was 400 ml. in all instances and the quantity of material extracted was such that the molar ratio of Na2C03 in solution to A1203 in the sinter was 2.0. The mixtures were shaken for 2 hours at the temperatures given in Table 7, filtered, and the residues washed with 100 ml. of hot water. In most of the experiments the combined extracts and washings were then boiled for 2 hours with seed charges consisting of 5 g. of synthetic sodalite. After removal of the sodalite by iltration the solutions were analyzed for A1203, Si02, and Na2O. The determinations of S102 Were made on samples large enough to conn tain 5-10 g. of A1203. extractions were made in which the extracts were not desilicized. The results of analyses of these extracts are referred to in Table 7 as Recoveries before desilicizing.

A number of duplicate A termined. Both 5 and 10 percent sodium carbonate solution were used, each containing 200 g. of NaCl per liter and the extraction temperature was 40 C. The quantities of material extracted were such that the molar ratio of Na2CO3 in solution to Al203 in the sinter was 2.0. The finelyground portion of the large-batch kaolin-lime burn was used in this research.

As shown in Figure 9 the optimum extraction time appears to be about 1 hour. Longer periods gave much reduced extractions for the 10 percent Na2C03 solution and slight reductions for the 5 percent solution.

The results of determinations which were made to establish the optimum molar ratio of NazCOa in solution to A1203 in the sinter, are shown in Figure 10. All of these solutions contained 200 g. of NaCl per liter and the extraction period was 1 hour at 40 C. Figure 10 shows that a molar Table 7.--Etractons of ground kaoZz'n-Zime sinter Composition of Recovenes Extracting Solutlm Before Desilicizing After Desilicizing Extraction 'Irltrpgcfff No' tion C. A1203 S'O .A1203 N Naf Na?! as' se' gfl gll' [l Per Cent Cent l] Per Cent Cent g' Cent g Cent 50 50 l5. 8 81 2. 14. 6 76 0. 2A 50 40 50 14. 6 76 98 0. 16 V50 80 50 15. 78 1. 9 14. 8 77 98 0. 1.0 50 120 50 14. 2 74 97 0. 10 50 160 5U 13. 3 69 0. 05 100 0 50 31. S 80 2. 4 30. 4 79 98 0. 27 100 160 50 27. 3 71 98 2. 0 2G. 9 70 97 0. l0 100 240 50 25. 0 65 09 100 1GO 85 l0. 3 26 0. 16 50 0 Boiling 13. 5 70 1. 100 160 Bolling 9. 5 24 0. 18

It will be noted from Table 'l that the initial extracts contained S102 in amounts corresponding toV 1.6 to 3.8 percent of the weight of A1203 in solution. The sodalite desilication reduced these values to 0.05-0.27 percent.

In these extracts somewhat better extractions of A1203 were obtained with a 5 percent Na2CO3 solution than with a 10 percent solution. Greater diierences were found later when the large-.batch lime-kaolin sinter was extracted.

As shown in Table 7 the purity of the A1203 with respect to S102, in the desilicized solutions, increased as the ratio of NaCl to NazCOa increased .but this was accompanied by diminished recoveries of A1203. Although the data of Table '7 indicates that the losses of soda (as Na2C03-l-NaCl) amounted to 2 to 3 percent of that present in the initial solution, it should be mentioned that these losses were calculated from the analyses of the solutions before and after extraction. Because of the small aliquots of solution which had to be taken, this method of analysis is not particularly sensitive. It was found later, by analysis of the residues remaining after extraction, that the losses of Na are actually much less than those reported in Table 7.

Satisfactory extractions of A1203 were obtained at both and 50 C. but when the extractions were carried out at 85 C. and at boiling, with NaCl present, greatly diminished recoveries or A1203 resulted. Subsequent extractions, therefore, were made at l0-50 C.

In order to establish the optimum time of extraction the data illustrated in Figure 9 was deratio of 2.5Na2CO3/A1203 gave the highest extractions. The reason for the decreased extractions obtained at a 3Na2C03/Al203 ratio is not known.

A number of analyses, of which the following is typical, were made of the solid residues from the above extractions:

The quantity of NazO contained in this residue represents 0.36 per cent of the total Na20 present in the extracting solution, which contained 50 g. of NazCOs and 200 g. of NaCl per liter. However, on the basis of the salts taken separately theloss of NaCl is but 0.05 percent, and that of Na2C03 about 1.5 percent, of the amounts present originally. In the process this loss in sodium carbonate might be made up, wholly or in part, by the amount of alkali usually found in kaolin or clay.

Our discovery of the very small loss of sodium chloride during the extraction, as indicated by the above analysis, indicates that the use of sodium nitrate in the process is also feasible. Our

researchhas1idis1sedthatthis'sait-appears to be* even more effective than sodium chloride in bringingabout desilication"` of the extracts.

The e'iec't ofthe presence of excess NaOH on the extractions at 40 C. is shown in Figure 11. The compositions of the solutions usedinthese experiments were' exactly the same' as those: described in the preceding paragraph except that they' contained a' quantityof added NaOH which corresponded toA percent of the Weight of Na2C03 in" the initial solutions. Poorer extractions'we'r'e obtained in all instances except in thoseivvhere the'rnolar Iratio of Na2C03 to Al203 vvas3.0. It will be noted that no maxima Were obtained in these extraction curves.

, A` pilot-plant extraction was made of the liniekaoliri sinter in order to prepare a considerable volume of solution which wouldbe more or less typical of the extracts obtained when a Spercent solutionof-'NazCOs was used. This extraction was made ori-'the unground material, before the most favorable conditions described in the precedingl sectionv hadvbeen worked out and the resulting solutionwas intended for desilication determinations only. A-lb bath of the sinter was treatodin' the digester atv 45 C. for 2 hours with a solution containing 5.75 lbs. of NaOH and 9.2 lbs. n The solution was separated from the rneans` ofthe lter press, and the residueit oroughly Washed'with warm water. A rec'overy` of w61 percent of the A1203 was obtained and the Si02/xl203'ratio in'solution was 1.03%. Useof the groundmaterial and choice of the optimum time andl composition of the extracting solution would have given a recovery of A1203 of about 80 percent. The combined extract andV wash hai-*dnthe following composition ing. perl.: Ahoi, 14g-2; S102, 0.144.; N201-I, 18.6; Na2co3,1o.9; NaCl, 68.2; and its pH was 12.7. Half-liter pol'- tionsY ofthis extract, both with and without added sodium' chloride,v were boiled with seed charges of synthetic sodalite, the seed then removed by ltration, andthe solutions analyzed. The results are summarizedA in Table 8.

T'ab'z 9L-Damnation of pilot-plant emmer of ungrouncl lime-kaolz'n sinter I-atio of SiO; to IT C1. /1 t SO A1203, Per Cent \a m g. ol 1 2 in {.)esufga Solution, Seed 1102125 of Solution,

10D \0 g./1. Added o mg g./1. in in A1203 Solution Precipitated When the quantity of seed addedwas 2 g. per liter the greatest lowering iIrSiOg concentration wasjobtained after 2 hours boiling, as shown in desilications Nos. 2 to 5 of Table 8. This quantity of seed is insuiTlcient, however, and, if the amountof sodalite added is increased to 10 g. per liter, the desilication proceeds further, as shown in No.A 5, Table 8. Increases in the concentration of NaCl in solution also bring about d'eoiasesin the silica concentration (Nos. 7-10', .Iable 8). The decreases in S102 associated with ing a sodalite-type compound'with'sod'a; alumina and silica)` in` any substantial amoi-inftL relative to thevveig'lit'ofI carbonate per liter in the" solution, will 'iectsigniiic'ant'decrease of'V the' silica to alumina ratioy of'V the d'esilicised solution.v AsV the improved result; is obtainedn`v continuously greater degree vsv/ithinc'srease4 inthe amount o'f Na'R.' even a complete saturation of the solution with Nata' may be employed if desired-and the: degree ofA saturation Vtol be' used'in any given case' will'be determinedby the point" to which it is desiredpto reduce the silica to alumina ratio. For" example, in the case'showvn in Fig'. 1j2.; fo r aV 5% duction of theirsilica'vtowell under-0.05% isefev fected with NaCl added to the solutiony inwabout four times that concentration l(about 200fgramsf per liter). Thusthoseskilldinithe art-'Will ap'- pr'eeiate that rfi-substantial 'benefit isettainedfand substantial use of our'invention is-made when the N azR is employed-inthefsolution insubstantial amount per liter compared to theamount of carbonate presen-t in thesolution.

In order to determinewhat effect thefcohoena trationrof vsodium hydroxide and thepl'jf would have on thev` desilic'ation of they extractja"` Vfew desilications lwerefr'r'iadel in whiohvarious' amounts of sodium hydroxide were'added'to the-solution;

After mea's'urinv'gwthe p`HA of the resulting'rsolutions,

590 mi. :portionswere boiled'fo'r 2`l'1'oursfvvith 5t g.

of sodalite', thefsodalite rer'noved'by ltration, andy the solutions analyzed Vfor Al2`v03`and S102.v The data, given in Table 9- and Figure 13,' s howtha'tj .desilication becorries lessl eificient' as'v theV sodium hydroxide concentration increases., A decomposition of the sodalite s'd, inthe presencleotthe solutions containing the larger amounts ofjNaOH, is indicated by an increase iny the A1203 concentration of theV desiliciz'edr solutions from 14.2"4 to 1415 g. per' 1.

r Table .9.-E'y1cctoj NaOH4 concentration ondesilication of sodium aluminate solution (sodium chloride in solution-68.2 g. per liter) Composition of Solution after D'eslcatoii Dcsilcation in H l y K .y No. solution P Grams'per liter Ratio 'of g/l e v SiOato .y A1203..L A1203 SiO2 Percent tions were unsuccessful because this treatmentv caused the precipitation of a. considerable part of the A1203, presumably as a" hydrated calciumaluminate.

ln other desilications the NaCl-I concentration" and the :pH were' decreased b'y passing C02 into the' solutions. Because of the low concentration of NaOH prevailing in the original extract it was diilcult to obtain significant lowering in pH and still prevent the precipitation of part of the alumina. However, a trend of increased erhciency of desiiication was indicated at the lower pH values.

An illustrative process for the extraction of alumina from kaolin or clay, employing the discoveries and methods described in the preceding sections, is set forth in the accompanying flow sheet (Figure 14). In this iiow sheet also no attempt has been made to portray all the various possible modifications of the separate steps such as the use of other concentrations or" the solutions or the use of other sodium salts than sodium chloride to depress the quantity of silica in solution but it will be understood that such modications are contemplated by our invention. Furthermore, the constituents of the kaolin other than alumina and silica have been omitted since they are discarded in the residue. The alumina and silica content of the laolin, given in the flow sheet, is approximately that of the kaolin referred to above. The values shown for quantities of material and composition of solutions are based on the results of both-laboratory extractions and small scale pilot-plant extractions and would be somewhat changed if a kaolin or clay of different composition than that shown in the flow sheet f;

were used, as will be understood by those skilled in the art.

The kaolin or clay, together with limestone, is ground and sintered in equipment such as that used in making Portland cement. Enough lime*- stone is added to make the composition of the resulting material, calculated on an ignited basis, fall approximately on the line joining dicalcium silicate and pentacalcium trialuminate in the CaO-Al2O3-Si02 diagram. Allowance is made for the lime and alumina required for the formation of 2CaO.TiO2 and 4CaO.Al2O3.Fe2O3 with any TiOz or FezOs in the raw materials.

The sintered material is nnely ground and then extracted at approximately 50 C. for a suitable time with sodium carbonate-sodium chloride solution. As above mentioned, the presence of the sodium chloride or other Narr reagent in this solution is caused by the iact that it, or some other sodium salt active in depressing the solubility oi` the sodium aluminum silicate complex, is required in a later stage of the process. A small reduction in the extraction oi alumina is caused by the presence of the NaIR (herein sodium chloride), which, however, cannot economically be avoided in a process recirculating the extracting solution.

After extraction of the alumina, the mixture is filtered and washed and the residue discarded or used as raw material for the manufacture of Portland cement.

The solution, including the washings, is next desilicized by the novel procedure upon which this process is based, namely the use of a sodium salt to precipitate the silica in solution in the form of an insoluble sodium aluminum silicate compound of the sodalite type. This procedure consists of boiling the solution with a seed charge of synthetic sodalite. The seed charge, together with the additional sodalite formed by the desilication of the solution is iiltered off and washed. The sodalite in excess of that required for seed for the next batch is returned to the rst stage of the process, allowance for its addition being made in calculation of the amount of limestone 24 to be used in the subsequent batch. By this means the small amounts of alumina and soda in the excess sodalite are recovered.

Alumina is precipitated as gibbsite from the desilicized solution. In the flow sheet this precipitation is shown as lcaused by the carbon dioxide in the scrubbed gases from the kiln. The precipitated gibbsite, A1203.3H2O, is filtered and washed with water low in silica. The solution and washings 4are used to scrub the flue gases from the kiln whereby evaporation of the excess water is caused and dust is removed from the gases. The evaporated and carbonated solution is returned to the extraction, together with enough sodium salt and sodium carbonate to make up the quantities of those materials discarded in the residue. The precipitated gibbsite is dried and calcined for electrolytic reduction to aluminum.

The alumina recovered from the process is shown as containing 0.06% of silica. However, this figure was used for illustration and does not constitute a limitation on the process. No diicuity has been experienced in reducing the silica to well below this -lgure merely by boiling the solution with sodalite seed in open containers in accordance with our1 invention.

As pointed out in connection with the method of Fig. 8, the desilication at atmospheric pressure possesses obvious advantages in the design of plant equipment over that required for desilication at elevated temperatures and pressures. However, i1" more complete desilication is required than can be obtained at atmospheric pressure the desilication may be carried out in autoclaves at higher pressures.

The general process just described may likewise be applied to low-grade bauxites, as illustratedly exemplified in Fig. l5. Again for case of reference, the materials actually used and procedure actually followed in our research are herein set forth.

Bauxite A, which has the composition given in Table 1, was selected for treatment. Other materials used included ground Indiana, limestone and technical or C. P. grades, of sodium carbonate, sodium chloride, and sodium hydroxide.

The ground bauxite was mixed with ground limestone in such amount that, after allowing for the CaO and A1203 required to form 4CaO.A1203.Fe203 and ZCaQTiOz with the Fe2O3 and TiOz present, the composition of the residue would lie on the join between ZCaOSiOz and 5CaO.3Al2O3 in the ternary system CaO-Al2O3-Si02. The mixture, in g. batches, was burned at 1290 C. for 1 hour and complete dusting of the product occurred on cooling. Its composition was as follows:

Per cent A1203 28.47 F6203 5.82 TiOz 1.24 SiOg 13.63 CaO 50.80

Total 99.96

Extractions of this material were made under conditions similar to those used in the extraction of the lime-kaolin sinter.

The results of some typical extractions of the bauxite-lime snter at 50 C. for 2 hours are shown in Table 10.

, Table, 10.--Ertractions-- atr` 50 C:- of vbauxite-lime sinterleif g.of..simfer per-liter of etractz'ng solution) '.Extractions 1 to 3-show that vthe'recovery of A1203 :increased asthe molar ratio yof' NazCOain solution *to A1203 inthe sinter increased `from 1.5 to 4.0.

"The recoveries of A1203 are high in spite of the Srelatively large 'concentrations kof NaCl. The

presence of small amounts of sodium hydroxide` inthe initial sc-lutlonswappears to have little eiTect I'on the extractions but'afconcentration of 50 g. of NaOH per liter (extraction 4,;Table gave a greatly reducedrecovery of A1203.

The extracts, which Were boiled with sodalite :according to the procedure heretofore described, Awere all effectively desilicized. However, the desilication was less Vcomplete in the case of extractions 2 and 4 thaniitwasv in extractions 1 and 3. In case 2 this is accounted vfor by the relatively loW concentration of-@NaCl in the extract and in case 4 bythe higher concentration of NaOH cou- Ypled with thelower concentration of AlzOaresultring from a poor recovery.

Anillustrative ow sheet of this embodiment of our process for the recovery of `alumina from ya low-grade bauxite is'shown in Figure 15. Since the methodv parallels thatusedfor kaolin, which lhas been discussed in considerable detail, the 110W sheet for the recovery of alumina from this highsilica bauxite should *be self-explanatory.

Although Figures 14and'1`5'showthe same percentage recovery of alumina Yfrom both the kaolin and bauxite, somey of 'the'data obtained inthis study indicates that a Vhigh recovery is to be expected from bauxites. This, coupled lwith the fact 'that bauxites-contain higherxpercentages of alumina than :kaolin .or clay,.would make the use of lbauxites in Athisprocess more i, advantageous. 41Iowever,`the larger .amount'of our research work 'was doneonlaolin 'because of the widespread and abundant occurrence of clays, Whereas the domes- -tic ysuppliesof even'low-gra'de bauxites are limited. It maybe 'advantageous'to increase the jyield in the process of Figs- 14 and r15 toregrind Ythe sinter after it leaves thekiln as indicatedin vthesinter treatmentin' FigfS.

From the'fcregoing descriptionthose skilled in `the art .will .perceive that our k.discovery may be applied to many,alkaline,.processesfor .extracting 4,alumina from clays ,and y.higl'l-silica Abauxtes.

YAs .abo-ye mentioned, .our Vinventionmay be .applied tothe ,recovery .of sodafand .alumina from. ,the red mud of the.ordinaryBayer process. 4In such application vthe :process in .gener-al .follows the Vtreatment of the sinter inr-Fig. 8, -with the chloride carbonate tailingsesolution (partially causticized or uncaustic-ized.) .delivered-.directly to ythe `extractor in .lieu .of the .digester .of i-Fig. -8, except that inthis case .the-process .builds up the- .NazCOs content ofthe solution, sothat in .lieu of' the addition of make-up NaaCOa, .a-removal lof excess NazCOs is provided, as by ,concentrating the solution ,suiiciently -to precipitate vtheexcess .of YNazCOa, followed by 1sufficient .dilution ofthe '26 'residual-solution "to condition l it for reuse inV the extraction step.

HSimilarly, our invention-may bev applied to improve the -desilicationin the -soda-lime process of extracting-alumina,- followingl generally the e vprocedure outlined in the treatment of -the Ysinter in Fig.- 8, with1 the tailings-solution -tuncausticized or only partially causticized)-v returned directlyto the extractor and. with=the soda (assoda ash, obtainabley as aforesaid) added together with the calciumcarbonate;--or -if desired the VChlor-idecarbonate tailings-solution, uncausticized-or part of it, may beconcentrated `to-a wet' slurry and burned `with the other materials in the kiln, the balance of the solution (uncausticized or partially causticized) ,or simply water,- being added in the extractor.

-Also, while our invention is'not limited in-its v application to'the lime-sinterprocesses, its application thereto yields animproved combination contemplated. as one of the specific applications of anginvention subordinate to -our--moregeneric discovery. n

While, ofcourse, vwe are not required to know or understand the theory of operation of our improvements,and while we are not to ybe boundK by our hypotheses as to why our observed improvements-are attained,-we here set forth for-conm- .r pleteness, what webelieve yto be the best explana- A phase, with which the solution is in equilibrium.

.1f no solid' phase is. present the concentration of tif) silica in solution is vnot -xed.

.Our present research has-discovered conditions under which the concentrationof silica in solution may be xedat low values by the formation of special types of silicates of low solubilities. *We have vdiscovered that formation of members of .the sodalitegroup ofminerals, or silicates of .a similar type, accompli-shes this desirable result.

These sodalite `typte Vcompounds appear towhav'e the general form'ula SNazOBAlgOsSiOaqNamR where q appears to approach-2, is the valence ofthe acid radical and R is an acid radical such as Cl,Br,NO3,-SO4, CO3, etc. `Although the-sodalite minerals are generally considered to be anhydrous, the compoundsY prepared in -t-he 'course of our .present research always contained some water of hydration and containedle'ss sodium salt than corresponded to the reported composition of the natural minerals. However the X-ray diffraction .patterns of all but the sulfatesalt were very similar to that of naturally-occurring sodalite. This fact may indicate that oursodalite precipitates, herein called sodalite-type compoundaware members of ra solid `solution series between a `hydrate and the respective'sodalit'e type end dnember, Vconsideredas anhydrous. -In

any event-the sodalite characteristic is observed .and wi'thit, the decreasedsilica solubility.

Winchell lhas stated that the mineral sodalite will release sodium chloride to ysolution When treated with boiling water. This indicates that the mineral is incongruentl'y soluble since its COD;y stituents do not dissolve vin the same molar'piof .portionsas are `present in the solid phase. The

.prescrit investigation-has resul-team the discovery that high concentrations in solution of the selected salt NaxR are required to stabilize the sodalite-type compounds and we have disclosed practical applications of this discovery. The increased stability of the compounds formed by employment of our invention is accompanied by a decrease in concentration of silica in the solutions with which they are in contact. On the other hand we have discovered that an increase in concentration of sodium hydroxide causes an increased dissociation of the sodalite-type compound with a resultant increase in the concentration of silica in solution. Therefore, the amount of silica remaining in solution was smallest when a low concentration of sodium hydroxide was accompanied by a high concentration of sodium chloride as in preferred examples of the practice of our method given above. As examples of the the very great effectiveness of the formation of sodalite in reducing the content of silica in alkaline solutions the following are noteworthy; Desilication of pilot-plant extract (Table 8). which contained 243 of NaCl per liter, reduced the concentration of silica to 0.0024 g. per liter. In other words. this corresponds to 2.4 parts of SiOz per million parts of solution. More recently when about 7 gallons 0f this solution were desilicized there remained but 1.5 parts of SiOz per million parts of solution. The concentration of silica in these alkaline solutions is even less than that of the water supply of Washington, D. C., where the SiOz ranges from about 4 to 6 parts per million.

We have further discovered that the rate of formation of the sodalite-type compounds increases rapidly with temperature. In order to avoid the formation of sodalite during extraction of the lime-kaolin and lime-bauxite sinters. with conseouent losses of alumina and soda, we apply this discovery by making our extractions of such sinters in the neighborhood of 50 C. or below, where the rate of sodalite formation is slow. The desilication of these extracts is likewise facilitated by our application of this discovery in that we carry out the desilication at boiling temperatures in the presence of sodalite seed and thus under conditions in which sodalite forms rapidly. For the same reason the initial extractions of the banxites in the embodiment of Figure -8 are made atrelativelv high temperatures, as from boiling to. say 150 C. in order to secure a rapid and complete formation of sodalite and thereby obtain extracts low in silica. Likewise the extraction of the residue obtained in our Figure 8 embodiment is carried out at a relatively low temperature. about 50 C. or less. to avoid formation of sodalite. and again, desilication of the extract thus obtained is subsequently accomplished bv raising the -temperature of the extract, as by boiling, in the presence of a seed charge, and thereby inducingr the formation of the sodalite type compounds.

In this connection it will be appreciated that a principal difference between the digestion process of Figure 8 and the extractions of Figures 14 and resides in the fact that in the former the sodalitic compound is separated in the red mud which is subiected to further` treatment, while in the latter the main bulk of the residue is separated prior to the final removal of silica brought about by the sodalite-type silica scavenger; therefore, in the former the digestion is carried out at a temperature, as boiling. facilitating formation of the sodalitic-type compound, while in the latter the extraction is conducted at 2b lOWQI tem perature and the temperature raised only when the silica scavenging step is reached, all pursuant to our present discovery.

Since all of the sodium salts employed in our research were effective in reducing the concentration of silica in sodium aluminate solutions it is to be anticipated that other sodium salts will behave similarly. These may include the salts of both strong and weak inorganic and organic acids. The acid radicals may be monoor polyvalent,E but the data summarized in Figures 2 to 7 indicates, as could not be predicted, that the monoyalent negative radical salts, as a class, are more advantageous than the polyvalent salts, and that the chloride and nitrate are more advantageous than the others of the group exemplified therein.

Furthermore, it will be appreciated from Table 5, above, that if it is desired to employ less salt (sodalitic salt former) than in the preferred embodiments, such practice of our invention may be employed to bring about a less complete reduction of the silica content of the solution, as, say to six to twenty-rive one-hundredths of a f percent of its alumina content, and this step may be combined with differential precipitation of most, say to 90%, of the alumina of the thus largely desilicized solution, as a hydrated alumina having a desirable silica-alumina ratio lower than that of the desilicized solution.

A further principal difference between the second extraction of Fig. 8 and those of Figs. 14 and 15 resides in the fact that the sinter in Fig. 8 contains its own soda, as Sodium aluminate, in addition to dicalcium silicate and is substantially free of calcium aluminate, while in the example of Figs. 14 and 15 the sinter is relatively free of soda and contains the calcium as calcium aluminate in addition to the dicalcium silicate. Thus in the embodiment of Figs. 14 and 15 enough NazCOa is supplied in the extractant solution to react with the calcium aluminate and reduce it to sodium aluminate and calcium carbonate; while in the embodiment of Fig. 8, little or no addition of NazCOs is reduired and that contained in the extractant solution is practically inert, and is not present in the large excess required to throw down an alkali-carbonate sodalitic compound in preference to the alkali chloride sodalitic compound, the same also being true in the modications referred to above.

Since the solubility of the sodalite is shown by our research (see Fig. 13) to increase with lncrease in the concentration of NaOH- it will be apparent that an improved desilication may be effected, with slightly less recovery of alumina. (see Fig. 1) by using a molar ratio of Na20 as NaOH to A1203 of approximately 15:1.0 in our embodiment of Fig. 8. and that the improved desilication may be eiected without decrease in the recovery of the A1203 by employing part of the CO?. (ue gases) to partially neutralize the NaOH between the digestor and the red mud lilter in the embodiment of Fig. 8, and between the rst iilter and the desilicizer boiler in the embodiments of Figs. 14 and 15 and in the sintertreatment portion of Fig. 8.

Further, we have set forth in detail one manner of synthesizing a representative example of a sodalitic-type compound suitable for seeding in our method. Initial seed may be obtained by employing a part of the sodalitic red mud obtained in the process of Fig. 8, notwithstanding that such material contains some inert ingredients, as the continual dilution of the initial seed charge with newly formed soda lite, and continual withdrawal of portions thereof for return to the sintering kiln effects, gradually a puriiication of the sodalitic compound circulating in the seed circuit. We prefer, however, to initiate the operation with a substantially pure seed charge of synthesized sodalitic-type compound, which as above indicated may be readily prepared by mixing silica gel with gibbsite and sodium hydroxide, both in a considerable excess relative to the theoretical quantities for combination, and with the sodalitic-compound-forming salt in large excess relative thereto, and inducing formation of the synthetic sodalitic compound by boiling, or preierably by autoclaving, in accordance with our discovery that the formation of the sodalitic compounds is accelerated with increase in temperature. Kaolins corresponding closely to kaolinite (Al2O3.2SiO2.2I-I2O) may be employed in lieu of the silica gel and part or all of the gibbsite in this synthesizing procedure with advantage. They yield a hner grained sodalitic seed and with this seed we have successfully reduced the silica content of sodium aluminate solution from 0.8% to less than 0.01% of the alumina content.

In preparing sodalite from kaolin for use in this process, it is especially desirable that the kaolin iirst be activated by heating for about one hour in the temperature range IOW-900 C. The

quantity of sodium hydroxide with which the r dehydrated kaolin is treated is preferably in moderate excess of that theoretically required to form sodalite and that of sodium. chloride in considerable excess of theoretical requirements.

As an example of the procedure, the preparation 1 of a batch of sodalite from kaolin was carried out as follows: The kaolin was first activated by heating it at '700 C. for one hour. A well-mixed batch of 200 g. of the dehydrated kaolin, 100 g. of sodium hydroxide, 150 g. of sodium chloride, and 400 cc. water was placed in an autoclave at 150 C. and maintained at that temperature for four days. The product was then removed, washed with hot Water, and dried at 110 C. Chemical Ignoring the FezOs-l-TiOz these values correspond to the following molar ratios:

in good agreement with the theoretical molar ratios of sodalite,

1.00Na: 1.00A120322.00S02 I 0.57NaC1 When the synthetic sodalite has been used in the desilication o1" sodium aluminate solutions of high pI-I (in the neighborhood of 13.5) some deterioration of the sodalite has been noted when it is used repeatedly. In such case the effectiveness of the sodalite as a desilicizing agent can be completely restored by boiling it for 1 or 2 hours with a strong (saturated or nearly saturated) sodium chloride solution prior to use in a subsequent desilication.

In conclusion it should be pointed out that the processes described in this specication do not require the utilization of those bauxites, kaolins, or clays which have extensive uses in the ceramic industries because of their low content of iron, and that it will be apparent to those skilled in the art that various modifications and applica-- tions of our discoveries in addition to those set forth as representative can be made without departing from our invention.

Having described illustrative embodiments exemplifying the practice of our invention, we claim:

1. A method of reducing the concentration of silica in sodium aluminate solutions, which comprises including in the solution a sodium salt capable of forming a sodalitic compound with soda, alumina and silica, separating any insoluble residue from the solution and heating the solution with an added seed-charge of such sodalitic compound to induce the withdrawal of silica from the solution inthe form of such sodalitic compound.

2. The method of separating silica from the alumina content of sinters of the lime-bauxite, lime-kaolin type, which consists in extracting such sinters with a sodium carbonate solution containing a substantial amount by weight, compared to the amount of carbonate in the solution, of another salt of sodium which is capable of forming a sodalitic compound with soda, alumina, and silica, at a relatively low temperature, at least approximately as low as about half way between room temperature and boiling, which is unfavorable to formation of such sodalitic compound; separating the liquid extract from the residue forme-d; and desilicizing the separated extract by heating the same, in the presence of a seed charge of such sodalitic compound, to induce the withdrawal of silica from solution in the form of such sodalitic compound.

3; A method according to claim 2, in which excess sodalitic compound formed, which is not employed for seed-charging subsequent batches, is included in the formation of the sinter to be extracted in such subsequent batches.

4. The method of activating deteriorated sodalitic-seeding material for use in desilicizing sodium aluminate solutions, which consists in boiling the seeding material for a period of about one to two hours in a strong solution of the sodium salt component of the sodalitic material.

EINAR P. FLINT. LEO SHARTSIS. LANSING S. WELLS.

REFERENES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 'Number Name Date 1,618,105 Pedersen Feb. l5, 1927 2,181,669 Scholder Nov. 28, 1939 2,283,849 Coles May 19, 1942 FOREIGN PATENTS Number Country Date 252,399 Great Britain June 9, 1927 REFERENCES Mellor: Inorganic and Theoretical Chemistry, vol. 6, pages 582-583. Longmans, London 1925. 

4. THE METHOD OF ACTIVATING DETERIORATED SODALITIC-SEEDING MATERIAL FOR USE IN DESILCIZING SODIUM ALUMINATE SOLUTIONS, WHICH CONSISTS IN BOILING THE SEEDING MATERIAL FOR A PERIOD OF ABOUT ONE TO TWO HOURS IN A STRONG SOLUTION OF THE SODIUM SALT COMPONENT OF THE SODALITIC MATERIAL. 